• Biological Molecules

  • Carbohydrates
  • Did you know?

    Did you know that carbohydrates, the main source of energy for the human body, are made of different types of sugar molecules? Depending on their molecular structure, some carbohydrates are broken down quickly in the body to release as energy; others need a special enzyme to digest; and still others cannot be digested at all by humans.

    Summary

    Our bodies are efficient chemical processing plants, breaking down nutrients to use and store for energy. This module introduces carbohydrates, an important macronutrient. It explains how different carbohydrates are used by plants and animals. Simple sugars and complex carbohydrates are identified, and their biochemical structures are compared and contrasted.

    • NGSS
    • HS-LS1.C2
    Key Concepts
    • Carbohydrates are a class of macronutrients that are essential to living organisms. They are the main energy source for the human body.
    • Carbohydrates are organic molecules in which carbon (C) bonds with hydrogen and oxygen (H2O) in different ratios depending on the specific carbohydrate.
    • Plants harvest energy from the sun and manufacture carbohydrates during photosynthesis. In a reverse process, animals break down carbohydrates during metabolism to release energy.
    • All carbohydrates are made up of units of sugar. There are two types of carbohydrates: simple sugars – the monosaccharides and disaccharides – and complex carbohydrates – the polysaccharides, which are polymers of the simple sugars.
    • Examples of complex carbohydrates are starch (the principal polysaccharide used by plants to store glucose for later use as energy), glycogen (the polysaccharide used by animals to store energy), and cellulose (plant fiber).

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  • Fats and Proteins
  • Did you know?

    Did you know that there are an estimated 100,000 different proteins in the human body? Proteins, one of the major nutrients required by our bodies, are large molecules made up of hundreds, even thousands, of amino acids combined in different ways. Fat is another essential nutrient, providing a reserve supply of energy, insulation and protection for the body, and structure for cells.

    Summary

    Fats and proteins are two of the major nutrient groups that our bodies need. This module provides an introduction to these two macronutrients. The basic chemical structure of fats as triglycerides is presented along with the purposes and types of fat. The module also introduces the amazing structure of protein molecules, including the peptide bond, and explains the purpose of proteins.

    • NGSS
    • HS-LS1.C2
    Key Concepts
    • In addition to carbohydrates, fats and proteins are the other two macronutrients required by the human body.
    • Fats, a subgroup of lipids, are also known as triglycerides, meaning their molecules are made from one molecule of glycerol and three fatty acids.
    • Fats in the body serve mainly as an energy storage system. They are also used as insulation to conserve body heat and protect internal organs, to form the main structural material in cell membranes, and to manufacture steroids and hormones to help regulate the growth and maintenance of tissue.
    • Fats are classified as saturated or unsaturated. Saturated fats contain no double carbon-carbon bonds in their fatty acid chains and tend to be solid at room temperature. Unsaturated fats contain double carbon-carbon bonds and are generally liquid at room temperature. Unsaturated fats can be either polyunsaturated (many double bonds) or monounsaturated (one or few double bonds).
    • Proteins are polymers of hundreds or even thousands of amino acids. Each protein has a different structure and performs a different function in the body. There are around 100,000 different proteins in the human body, each of which is made up of combinations of only 20 amino acids.
    • Enzymes are proteins that help to carry out specific chemical reactions in the body.

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  • Biological Proteins
  • Did you know?

    Did you know that the human body contains an estimated 100,000 different proteins, all due to the numerous ways that only 20 amino acids can combine? Proteins are large molecules made up of hundreds, even thousands, of amino acids combined in different ways.

    Summary

    This module explores how proteins are polymers composed of building blocks called amino acids. Using the historic research of Frederick Sanger on insulin as a starting point, the complex structures of proteins, due to molecular bonds like the disulfide bridge and the peptide bond, are explained.

    Key Concepts
    • Proteins are vital components to nearly every biological process.
    • Proteins are polymers composed of building blocks called amino acids, of which life on Earth uses just twenty.
    • Molecular bonds determine the structures of amino acids and proteins. Peptide bonds link amino acids together in a chain; disulfide bridge bonds hold proteins together.
    • Using techniques like electrophoresis and chromatography, Frederick Sanger discovered that proteins were built of specific amino acid sequences and that changing the sequence would make it a different protein.
    • Proteins can have four types of structures: (1) Primary, the sequence of amino acids, (2) Secondary, hydrogen bonds among the strands of amino acids form beta sheets or alpha-helixes, (3) Tertiary, the three-dimensional, twisted structure based on bonding interactions between amino acid strands, and (4) Quartnerary, the complex structure made up of multiple folded subunits.

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  • Blood Biology I
  • Did you know?

    Did you know that in the early days of blood transfusions, more people died than survived them? All blood looks pretty much the same to the naked eye, but blood that is lifesaving for one person may be deadly to another. Transfusions became a safe medical procedure only when scientists came to understand the complexity of blood components and different blood types.

    Summary

    Knowledge of blood components brought about a revolution in surgery through safe transfusion. The module traces the development of our understanding of blood over centuries, beginning in 1628 with English physician William Harvey's breakthrough research on circulation. With a focus on early 20th-century experiments by Austrian researcher Karl Landsteiner, the module shows how the identification of clotting factors, blood types, and antigens was critical to medical science. Whole blood, plasma, serum, and different types of blood cells are defined.

    Key Concepts
    • Blood is a complex fluid with many different components, but can be divided into solids (red blood cells, white blood cells, and platelets) and liquid (plasma).
    • Blood plasma includes clotting factors (agents that help to form blood clots) and when these are removed, the remaining liquid is known as serum.
    • The main cellular components of blood are: red blood cells (erythrocytes), white blood cells (leukocytes), and platelets (thrombocytes).
    • The Austrian researcher Karl Landsteiner studied agglutination, or clumping together of blood cells with certain antigens. Based on his findings, he proposed that there were three types of blood (A, B, O) and later added a fourth type (AB).
    • Antibodies are proteins produced by plasma cells, a type of B-cell lymphocyte, and are present in the blood serum. These antibodies are important for blood transfusion, since the blood type of a patient and the type of antibodies present in the donor’s blood will determine whether or not it agglutinates or clumps.

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  • Lipids
  • Did you know?

    Did you know that studying lipids can help us understand and treat medical conditions such as heart disease, hormone disorders, multiple sclerosis, and many others? Lipids are necessary for the structure of all living cells. Their chemical composition allows them to have many important functions, from storing energy to regulating metabolism to helping the fur coat of sea otters repel water.

    Summary

    Fats, oils, waxes, steroids, certain plant pigments, and parts of the cell membrane – these are all lipids. This module explores the world of lipids, a class of compounds produced by both plants and animals. It begins with a look at the chemical reaction that produces soap and then examines the chemical composition of a wide variety of lipid types. Properties and functions of lipids are discussed.

    • NGSS
    • HS-LS1.C2
    Key Concepts
    • Lipids are a large and diverse class of biological molecules marked by their being hydrophobic, or unable to dissolve in water.
    • The hydrophobic nature of lipids stems from the many nonpolar covalent bonds. Water, on the other hand, has polar covalent bonds and mixes well only with other polar or charged compounds.
    • Fats and oils are high-energy molecules used by organisms to store and transfer chemical energy. The distinct structures of different fat molecules gives them different properties.
    • Phospholipids are specialized lipids that are partially soluble in water. This dual nature allows them to form structures called membranes which surround all living cells.

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  • Cell Biology

  • Discovery and Structure of Cells
  • Did you know?

    Did you know that the human body consists of trillions of individual cells and 200 distinct types of cells? Human cells range in size from 1/12,000 of an inch (a few micrometers) to over 39 inches (more than a meter) long. All living things are made of cells, but in spite of vast differences in size, shape, and function, these building blocks of life share remarkable similarities.

    Summary

    Cells are the basic structural and functional unit of life. This module traces the discovery of the cell in the 1600s and the development of modern cell theory. The module looks at similarities and differences between different types of cells and the relationship between cell structure and function. The Theory of Universal Common Descent is presented along with evidence that all living things on Earth descended from a common ancestor.

    Key Concepts
    • Cells are the basic structural and functional unit of all living things and contain inheritable genetic material.
    • The activity of a cell is carried out by the sub-cellular structures it possesses.
    • Cells possess an outer boundary layer, called a cell membrane, cytoplasm, which contains organelles, and genetic material.
    • There is considerable variety among living cells, including the function of membranes and subcellular structures, and the different types of functions the cells carry out, such as chemical transport, support, and other functions.

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  • Membranes I
  • Did you know?

    Did you know that Benjamin Franklin’s 1774 experiments with pouring oil onto a pond of water was an early step in gaining a scientific understanding of cell membranes? Cell membranes were thought to be passive barriers until the 1960s, but we now know that they are active and responsive structures that serve a critical function as gatekeepers and communicators.

    Summary

    Cell membranes are much more than passive barriers; they are complex and dynamic structures that control what enters and leaves the cell. This module explores how scientists came to understand cell membranes, including the experiments that led to the development of the fluid-mosaic model of membrane structure. The module describes how the components and structure of cell membranes relate to key functions.

    • NGSS
    • HS-C6.2, HS-LS1.A1
    Key Concepts
    • The outer layer of a cell, or a cell membrane, is a complex structure with many different kinds of molecules that are in constant motion, moving fluidly throughout the membrane.
    • Cell membranes form selective barriers that protect the cell from the watery environment around them while letting water-insoluble molecules like oxygen, carbon dioxide and some hormones pass through.
    • Most of the cell membrane is formed by phospholipids that have a unique structure that causes them to self-arrange into a double layer that is hydrophobic in the middle and hydrophilic on the outside.

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  • Membranes II
  • Did you know?

    Did you know that the absence of one tiny amino acid in cell membranes causes Cystic Fibrosis, a life-threatening disease? And a common aliment, heartburn, is treated with medicine that slows down the rate at which protons are pumped across cell membranes into the stomach. Studying how molecules travel across plasma membranes (cell membranes) is the key to understanding and treating many medical conditions.

    Summary

    For living things to survive, different molecules need to enter and leave cells, yet cell membranes serve as a barrier to most molecules. Fortunately, all living cells have built-in transporters that allow water, glucose, sodium, potassium, chloride, and other molecules to cross the plasma membrane. This module looks at how passive and active transporters work. It highlights the importance of the study of cell membranes by looking at advances in treating cystic fibrosis and common digestive ailments as well as the development of effective pain relievers.

    • NGSS
    • HS-C6.2, HS-LS1.A1, HS-LS1.C3
    Key Concepts
    • Whether or not a molecule is able to pass easily, or at all, into or out of a cell is largely dependent on its charge and solubility in water.
    • The plasma membrane serves as a semi-permeable barrier to the cell. Only uncharged, non-polar molecules are able to pass into or out of the cell without aid.
    • All plasma membranes possess transporters to help move molecules from one side of the membrane to the other. These transporters can be active (pumps) or passive (channels) and are sometimes regulated by gates.
    • The lack of a specific transporter can interrupt cellular functions and cause diseases like cystic fibrosis.
    • Research into pain relievers provided insight into the most important and universal transporter in the human body, the sodium-potassium pump.

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  • Cellular Organelles I
  • Did you know?

    Did you know that “survival of the fittest” is not the only explanation for the success of a species over time? Cooperation can be just as important when it comes to how species adapt in order to survive. According to Lynn Margulis, who proposed that modern-day mitochondria and chloroplasts evolved through endosymbiosis, “Life did not take over the globe by combat, but by networking.”

    Summary

    Evolution isn't always about competition. It can also be about cooperation, as is the case with the development of chloroplasts and mitochondria from free-living bacteria. This module explains the theory of endosymbiosis along with its origins. Convincing evidence in support of the theory is presented. The evolution of the nucleus and other organelles through invagination of the cell membrane is also discussed.

    • NGSS
    • HS-C6.1, HS-C6.2, HS-LS1.A1, HS-LS1.A3
    Key Concepts
    • One of the main differences between eukaryotic cells and prokaryotic cells is the presence of a nucleus and other membrane-bound organelles.
    • Chloroplasts and mitochondria have specialized roles in producing energy for the cell and have several unique features including some of their own DNA. Because of this, scientists believe that both of these organelles originated through endosymbiosis when one small cell began to live inside a larger one.
    • Membrane-bound organelles evolved as folds of the plasma membrane; this allowed these cells to establish compartments with different environments appropriate for the specific function that the organelle performs.

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  • Cell Division I
  • Did you know?

    Did you know that every organ and tissue in your body was formed as the result of individual cells making copies of their DNA and separating themselves into two identical cells? From experiments in the 1870s to research more than 100 years later, scientists have made fascinating discoveries about the complex series of events that allow the cells in plants and animals, including humans, to grow and sustain life.

    Summary

    Cell division is an enormously complex process that must go on millions and millions of times during the life of an organism. This module explains the difference between binary fission and the cell division cycle. The stages of cell division are explored, and research that contributed to our understanding of the process is described.

    • NGSS
    • HS-C6.2, HS-LS1.B1
    Key Concepts
    • Most of the cells that make up higher organisms, like vertebrate animals and flowering plants, reproduce via a process called cell division.
    • In cell division, a cell makes a copy of its DNA and then separates itself into two identical cells – each with its own copy of DNA enveloped inside a nucleus.
    • The term mitosis refers specifically to the process whereby the nucleus of the parent cell splits into two identical nuclei prior to cell division.

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  • Cell Division II
  • Did you know?

    Did you know that there is a huge variation in the number of chromosomes in living things? While humans have 46 chromosomes and dogs have 78, one kind of ant has only 2 chromosomes and a type of protozoan has nearly 16,000! But what all these life forms have in common is that their genetic code is copied from cell to cell thanks to the process of mitosis, whereby the nucleus of a cell splits into two before the cell divides.

    Summary

    Beginning with the discovery of mitosis, the module details each phase of this cell process. It provides an overview of the structure of cell components that are critical to mitosis. The module describes Clark Noble’s experiments with the Madagascar Periwinkle, which led to the discovery of an effective cancer treatment drug. The relationship between mitosis and cancer is explored as is the mechanism by which anti-cancer drugs work to slow down or prevent cell division.

    • NGSS
    • HS-C6.2, HS-LS1.A1, HS-LS1.B1
    Key Concepts
    • The term mitosis refers specifically to the process whereby the nucleus of a eukaryotic cell splits into two identical daughter nuclei prior to cell division.
    • Mitosis is a cyclical process consisting of five phases that feed into one another: prophase; prometaphase; metaphase; anaphase; telophase.
    • The rate at which mitosis occurs depends on the cell type. Some cells replicate faster and others slower, and the entire process can be interrupted.
    • Chromosomes are made of a material called chromatin, which is dispersed throughout the cell nucleus during interphase. During mitosis, however, the chromatin condenses making individual chromosomes visible under an ordinary light microscope.

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  • Absorption, Distribution, and Storage of Chemicals
  • Did you know?

    Did you know that most chemicals we come into contact with—including the food we eat—must pass through a complex system of cell membranes before they can enter the bloodstream? There are many different ways that chemicals enter the body, depending on the type of chemical and the part of the body with which it comes into contact.

    Summary

    In order for many medicines to work, the chemicals must move from the outside environment into the body. This module discusses the different mechanisms by which chemicals cross the cell membrane and the factors that influence this process. In addition to introducing biological absorption, the module explains how chemicals are stored and distributed within the body.

    Key Concepts
    • Chemicals can enter the human body by several methods, but most must pass through living cell membranes before entering the bloodstream.
    • The cell membrane consists mainly of phospholipids and proteins in the form of a lipid bilayer.
    • Mechanisms for moving chemicals through the cell membrane include: passive diffusion, facilitated diffusion, active transport, and endocytosis.
    • Factors such as human anatomy and chemical structures affect the movement of chemicals in the body.

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  • Energy in Living Systems

  • Energy Metabolism I
  • Did you know?

    Did you know similar to the way cars are manufactured, chemical compounds in living cells are built up, broken down, and moved around in assembly-line fashion? In living organisms, a series of reactions is needed to get energy from food molecules. One such important chemical pathway is the circular assembly line known as the Krebs cycle.

    Summary

    Food fuels our bodies, but how does our body convert food molecules into usable energy? This module looks at glycolysis and the Krebs cycle, two important stages of cellular respiration, the process by which cells harvest energy from food. It highlights the work of Sir Hans Adolf Krebs and his focus on cyclic pathways as he discovered the main biochemical pathway for breaking down fuel to produce energy.

    • NGSS
    • HS-C5.2, HS-LS1.C4
    Key Concepts
    • In a cell, chemical compounds are put together, taken apart, and moved around through pathways that resemble moving assembly lines.
    • The main types of biological macromolecules that cells use for fuel are sugars, fats, and proteins.
    • The main biochemical pathway where the breakdown of biological fuels comes together is called the Krebs cycle. Named for its discoverer, Sir Hans Adolf Krebs, this pathway is like a circular assembly line.

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  • Energy Metabolism II
  • Did you know?

    Did you know that the energy in chemical compounds is found in tiny electrons? The electron transport chain is like an assembly line inside of cells that harnesses high-energy electrons so they can be used to make ATP, the energy that organisms need to survive. When Peter Mitchell proposed the way that ATP is made inside cells, other scientists made fun of him – until he was eventually proved correct and won the Nobel Prize in Chemistry.

    Summary

    ATP is the main energy currency of living cells. This module answers the question of how most ATP is generated. A look at two important compounds, NADH and FADH2, reveals their important role in the production of ATP. The module explains the workings of the electron transport chain, which provides high-energy electrons to fuel the ATP-producing process called oxidative phosphorylation.

    • NGSS
    • HS-C5.2, HS-LS1.C4
    Key Concepts
    • Adenosine triphosphate (ATP) is the main energy currency of the cell. It is generated from a similar compound, ADP, using energy harnessed from cellular fuels, such as sugars, fats, and proteins.
    • The amount of ATP generated directly during glycolysis (the breakdown of the sugar glucose) is small compared with amount of energy contained within glucose.
    • The energy held by ATP and other energy-holding chemical compounds is contained in electrons. By moving electrons, different molecules move energy around the cell.
    • Two specialized energy currency compounds, NADH and FADH2, are vital to the movement of high-energy electrons from cellular fuels like glucose to an assembly-line system of enzymes called the electron transport chain.
    • Located inside mitochondria, the electron transport chain harnesses energy from NADH and FADH2 to power a process called oxidative phosphorylation, which generates large amounts of ATP. Oxidative phosphorylation requires oxygen.

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  • Photosynthesis I
  • Did you know?

    Did you know that the oxygen we breathe is a waste product? Of photosynthesis, that is. Through this remarkable process, plants capture energy from sunlight and produce the sugars that provide sustenance to nearly every living thing on Earth along with the oxygen we need to survive.

    Summary

    Through photosynthesis, plants harvest energy from the sun to produce oxygen and sugar, the basic energy source for all living things. This module introduces photosynthesis, beginning with experiments leading to its discovery. The stages of photosynthesis are explained. Topics include the role of chlorophyll, the action spectrum of photosynthesis, the wavelengths of light that drive photosynthesis, light-harvesting complexes, and the electron transport chain.

    • NGSS
    • HS-C5.2, HS-LS1.C1
    Key Concepts
    • Photosynthesis is a process by which an organism converts light energy from the sun into chemical energy for its sustenance.
    • Photosynthesis occurs in plants, algae, and some species of bacteria.
    • In plants, chloroplasts contain chlorophyll that absorbs light in the red and blue-violet regions of the spectrum.
    • Photosynthesis occurs in two stages: the light-dependent stage that occurs in the thylakoid membrane of the chloroplast and harvests solar energy, and the light-independent stage that takes that energy and makes sugar from carbon dioxide.

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  • Evolutionary Biology

  • Origins of Life I
  • Did you know?

    Did you know that it is much easier to determine when life appeared on Earth than how life came to exist? Evidence points to life on Earth as early as 3.8 billion years ago, but the question of how life came to be has puzzled scientists and philosophers since prehistoric times. In the 1950s, scientists successfully created biological molecules by recreating the atmosphere of primordial Earth in a bottle and shocking it with lightning. This and other experiments give clues to the origins of life.

    Summary

    Since prehistoric times, people have pondered how life came to exist. This module describes investigations into the origins of life through history, including Louis Pasteur’s experiments that disproved the long-held idea of spontaneous generation and and later research showing that the emergence of biological molecules from a nonliving environment – or abiogenesis – is not only possible, but likely under the right conditions.

    • NGSS
    • HS-C4.2, HS-C4.3, HS-ESS2.E1
    Key Concepts
    • Theories about the origins of life are as ancient as human culture. Greek thinkers like Anaximander thought life originated with spontaneous generation, the idea that small organisms are spontaneously generated from nonliving matter.
    • The theory of spontaneous generation was challenged in the 18th and 19th centuries by scientists conducting experiments on the growth of microorganisms. Louis Pasteur, by conducting experiments that showed exposure to fresh air was the cause of microorganism growth, effectively disproved the spontaneous generation theory.
    • Abiogenesis, the theory that life evolved from nonliving chemical systems, replaced spontaneous generation as the leading theory for the origin of life.
    • Haldane and Oparin theorized that a "soup" of organic molecules on ancient Earth was the source of life's building blocks. Experiments by Miller and Urey showed that likely conditions on early Earth could create the needed organic molecules for life to appear.
    • RNA, and through evolutionary processes, DNA and the diversity of life as we know it, likely formed due to chemical reactions among the organic compounds in the "soup" of early Earth.

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  • Origins of Life II
  • Did you know?

    Did you know that scientists don’t need time travel to mimic conditions on Earth 4 billion years ago? Scientists who want to experiment in an environment like that of primordial Earth need only to visit volcanoes, which have chemical conditions similar to those on Earth long, long ago. That’s just what chemist David Deamer did in his research into the origins of life. Just as had happened in the lab, Deamer’s volcano experiments produced a necessary step toward the formation of living matter.

    Summary

    Building on earlier experiments showing how life’s chemical building blocks could form from nonliving material on early Earth, this module explores theories on the next steps needed for life. These include the formation of long polymers, which then fold into complex macromolecules. The module describes experiments in an environment like that of primordial Earth, resulting in the spontaneous emergence of phospholipids, which could form into membranes, paving the way for RNA duplication and the eventual emergence of living cells.

    Key Concepts
    • For life to occur, smaller molecules must join together and form polymers, which then fold into complex shapes. These large molecules are called macromolecules.
    • Simple membranes made of lipids may have served as nature’s test tubes, providing the enclosed environments necessary for RNA enzymes to develop.
    • The possible ancestor to living cells, liposomes, may have been created from phospholipids formed from the gases of Earth’s primeval atmosphere or from free fatty acids delivered to ancient Earth via meteorites.
    • To trigger abiogenesis, a system of molecules would need to develop the ability to copy themselves using polymers.
    • Protocells made of liposomes that exchanged fatty acids between their membranes possibly absorbed RNA enzymes and made copies of themselves, leading to the evolutionary development of living cells.

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  • Charles Darwin I
  • Did you know?

    Did you know that the theory of evolution did not begin with Charles Darwin? The idea of evolution was part of Western thought for more than 2,000 years before Darwin changed the world with his legendary book On the Origin of Species.

    Summary

    The experiences and observations of Charles Darwin significantly contributed to his theory of evolution through natural selection. This module explores those influences and describes evolution as a force for biological change and diversification. The first in a series, it details how the theory challenged the cultural mindset of the time, including the effect of his major works: On the Origin of Species by Means of Natural Selection and Sexual Selection and the Descent of Man.

    • NGSS
    • HS-C1.1, HS-LS4.A1
    Key Concepts
    • Charles Darwin played a key role in supporting and explaining the theory of evolution through natural selection.
    • Darwin's skills of observation and ability to record data accurately allowed him to create a comprehensive model of the mechanism by which evolution occurs.
    • The theory of evolution through natural selection explains how all forms of life are related to one another genealogically, and emphasizes that variation within a species is the root for evolutionary change.

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  • Charles Darwin II
  • Did you know?

    Did you know that Darwin's experience with his ten children fueled his thinking about evolution? He theorized that some human behaviors, such as a young child's selfishness, were based upon instincts that were adaptations. These natural differences that always exist among individuals are at the heart of the principle of natural selection as the engine of evolutionary change.

    Summary

    The second in a series discussing the work of Charles Darwin, this module takes a deeper look into the processes that led to Darwin's theory of natural selection and examines specific mechanisms that drive evolutionary change. Key points on which the idea of natural selection rests are outlined. Examples from Darwin's personal life shed light on his thinking about change within a species and the "struggle for existence."

    • NGSS
    • HS-C7.2, HS-LS4.B1, HS-LS4.C1
    Key Concepts
    • Variation within a species increases the likelihood that at least some members of a population will survive under changed environmental conditions.
    • The common characteristics of individuals within a population will change over time, as those with advantageous traits will come to be most common or widespread.
    • While evidence of evolution by natural selection exists, its effects cannot be predicted.

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  • Charles Darwin III
  • Did you know?

    Did you know that Charles Darwin preferred the phrase descent with modification over the simpler term evolution? In his groundbreaking book On the Origin of Species, Darwin chose his words very carefully. "Evolution" was used in different ways at the time, and Darwin wanted to convey the important concept that life forms descended from a common ancestor.

    Summary

    Our understanding of the term evolution has changed significantly since Darwin's time. This module explains how Darwin's work helped to give evolution the meaning it has today. It details the concept of "descent with modification" that Darwin described with the one figure originally included in Origin of Species. The module discusses how this model revolutionized scientific thinking about the similarities and differences between and within species, laying the foundation for our current understanding of biodiversity.

    • NGSS
    • HS-C2.1, HS-C2.2, HS-LS4.B1, HS-LS4.B2, HS-LS4.C1, HS-LS4.C2
    Key Concepts
    • Darwin's theory of Descent with Modification shows how as organisms reproduce, slight changes create variation, which could lead to new species over time.
    • Darwin provided the first model that could logically account for biodiversity, explaining lineage and the small variations that distinguish one species from another, similar-looking one.
    • Darwin's work radically changed thinking regarding the Scale of Nature, a model that suggested that some species were naturally inferior to one another, and showed species evolved in response to environmental pressures, not because of some hierarchy of order.

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  • Adaptation
  • Did you know?

    Did you know that there is a species of moth with a 12-inch long nectar-gathering tongue? Not by coincidence, this moth feeds on and pollinates a kind of orchid that has an 11-inch long nectar-producing tube. Nature abounds with examples of plants and animals that have adapted to their environment over time to ensure the survival of the species.

    Summary

    This module introduces the concept of evolutionary adaptation. It follows the development of Charles Darwin's ideas on how species adapt to their environment in order to survive and reproduce. The difference between adaptation and natural selection is explained. With a look at penguins and other examples from nature, the module explores the processes that influence the diversity of life.

    • NGSS
    • HS-C6.2, HS-LS4.C2
    Key Concepts
    • Natural selection is the mechanism that explains how organisms change.
    • The structure of an organism and many of its features are directly related to the environment in which it lives.
    • Numerous environmental mechanisms, both naturally occurring and man-made, influence adaptive evolution.

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  • Taxonomy I
  • Did you know?

    Did you know that people started classifying living things as early as 300 BCE? But our modern classification system officially began in the 18th century when Carolus Linneaus listed every plant and animal species known in the world – more than 12,000 in all. He produced one of the great works in the history of science, Systema Naturae, which we still use today.

    Summary

    Modern taxonomy officially began in 1758 with Systema Naturae, the classic work by Carolus Linnaeus. This module, the first in a two-part series on species taxonomy, focuses on Linnaeus’ system for classifying and naming plants and animals. The module discusses the contribution of diverse cultures to the development of our modern biological classification and describes the historical development of a scientific basis for classifying species.

    • NGSS
    • HS-C1.5, MS-LS4.A2
    Key Concepts
    • Under Linnaeus's system, every species is known by a unique Latin-sounding genus and species name that distinguishes it from other species.
    • Linnaeus's work organized organisms into logical classes based on their appearance and characteristics, and thus provides a basis for comparing different species.

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  • Taxonomy II
  • Did you know?

    Did you know that Tyrannosaurus rex could have been called Manospondylus gigas? The rules of scientific nomenclature usually dictate that when more than one name for a species is discovered has been given, the older one prevails. Luckily, common sense won out in the case of T. Rex, and this most famous dinosaur was allowed to keep the newer name that both scientists and the general public had become familiar with.

    Summary

    Carolus Linnaeus, the “father of taxonomy,” developed a uniform system for naming plants and animals to ensure that each species has a unique name. This module outlines rules of forming two-term taxonomic names according to genus and species. The module gives examples of naming controversies and describes how they were resolved, including by bending the rules in regard to certain famous beasts.

    • NGSS
    • HS-C1.5, MS-LS4.A2
    Key Concepts
    • The system of binomial nomenclature was Linnaeus' response to the need of a clear, distinct naming of species that would be recognized around the world and reduce the chance of one species being known by multiple names.
    • Scientific names are always written in italics, with the genus capitalized and the species lowercase, and should sound as though they are Latin.

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  • Genetics

  • Mendel and Inheritance
  • Did you know?

    Did you know people used to believe that fully formed miniature versions of offspring were contained in sperm cells? Early theories of reproduction were later disproven, but inheritance patterns remained a mystery until Gregor Mendel performed his groundbreaking experiments with pea plants in the 1800s.

    Summary

    This module describes the experiments that resulted in Mendel's Laws of Inheritance. A look at specific traits in pea plants over generations shows how Mendel's research methods resulted in an understanding of dominant and recessive genes. Partial dominance is also discussed.

    • NGSS
    • HS-C1.4, HS-C1.5, HS-LS3.A1, HS-LS3.B2, MS-LS4.B2
    Key Concepts
    • Mendel determined that an organism inherits two copies of the genetic material that determines an individual's physical traits, one copy coming from each the male and female parent.
    • Mendel observed that for each trait, sometimes what is inherited from one parent masks what is inherited from the other. He called the hidden trait recessive and the expressed trait dominant.
    • Since the time of Mendel, other scientists have observed that not all traits are inherited with the simple dominant-recessive pattern; incomplete dominance and co-dominance can result in a variety of phenotypes for some traits.

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  • Mendel and Independent Assortment
  • Did you know?

    Did you know that Gregor Mendel is known as the “Father of Genetics,” and yet his work was largely ignored by scientists during his lifetime? It was only when three scientists rediscovered Mendel’s work nearly 35 years after it was published that people came to appreciate its implications for the scientific understanding of inheritance.

    Summary

    The power of Mendel’s scientific approach can be seen in the research that led to his Second Law. This module, the second in a series, provides details on Mendel's work with dihybrid crosses and independent assortment. The module describes tests that confirmed Mendel’s ideas about the random and independent segregation of genetic factors.

    • NGSS
    • HS-C1.4, HS-C1.5, HS-LS3.A1, HS-LS3.B2, MS-LS4.B2
    Key Concepts
    • Genetic markers randomly and independently segregate into a parent’s gametes, some of which are dominant over others.
    • The cross of two organisms that each possess multiple heterozygous pairs is called a dihybrid cross.
    • Dihybrid crosses result in a trait expression ratio of 9:3:3:1 – 9 with both traits dominant, 3 with trait one dominant and trait two recessive, 3 with trait one recessive and trait two dominant, and 1 with both traits recessive.

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  • DNA I
  • Did you know?

    Did you know that one of the most important discoveries in biology was made while a British army medical officer was trying to develop a vaccine for pneumonia after World War I? Although a vaccine for pneumonia still does not exist, Frederick Griffith discovered “transformation.” This means that organisms can be genetically reprogrammed into a slightly different version of themselves.

    Summary

    This module is the first in a series that discusses the discovery, structure, and function of DNA. Key experiments are discussed: from Griffith’s discovery of genetic “transformation” to Avery, MacLeod, and McCarty’s determination of the “transforming agent” to confirmation by Hershey and Chase of DNA rather than protein as the genetic material.

    • NGSS
    • HS-C6.1, HS-C6.2, HS-LS1.A2
    Key Concepts
    • It required numerous experiments by many scientists to determine that DNA, and not protein, is the genetic material on which life is built.
    • DNA can be “transformed,” or genetically re-programmed, into a slightly different version of itself.

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  • DNA II
  • Did you know?

    Did you know that the precise combinations of just four nitrogen bases form the billions of nucleotides that make up our own unique DNA molecules? The information stored in the base sequence of a single DNA strand stores all of the genetic information in your body and gives us our individual genetic traits.

    Summary

    Exploration of the structure of DNA sheds light on fascinating properties of the molecule. This module, the second in a series, highlights major discoveries, from the parts of a nucleotide - the building blocks of DNA - to the double helix structure of the DNA molecule. The module describes scientific developments that led to an understanding of the mechanism by which DNA replicates itself.

    • NGSS
    • HS-C6.1, HS-C6.2, HS-LS1.A2, HS-LS3.B1
    Key Concepts
    • DNA consist of two strands of repeating units called nucleotides; each nucleotide is made up of a five-carbon sugar, a phosphate group, and a nitrogen base.
    • The specific sequence of the four different nucleotides that make up an organism's DNA gives that organism its own unique genetic traits.
    • The four nitrogen bases are complementary – adenine is complementary to thymine, cytosine is complementary to guanine – and the pairs form hydrogen bonds when the 5'/3' ends of their attached sugar-phosphate groups are oriented anti-parallel to one another.

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  • DNA III
  • Did you know?

    Did you know that DNA in a human body must make exact copies of itself not merely thousands of times, not millions of times, and not even billions of times, but staggeringly, more than a trillion times? Whether a yeast, a bacterium, or a human cell, every living cell must be able to copy all of its DNA with amazing accuracy.

    Summary

    In the field of molecular biology, scientists examine how DNA encodes all the complexities of living things. This third module in the DNA series focuses in the process by which DNA is replicated. The module describes the DNA synthesis assay, where scientists were able to replicate DNA in a test tube. Advancements in understanding the features and properties of DNA replication are discussed.

    • NGSS
    • HS-C6.1, HS-C6.2, HS-LS3.B1
    Key Concepts
    • Once the structure of the DNA molecule was discovered, scientists could immediately envision a possible copying mechanism based on the rules of nucleotide-base pairing.
    • In order to study and observe DNA replication more directly, scientists in the 1950s devised techniques to perform DNA replication in a test tube, called the DNA synthesis assay.
    • By using the DNA synthesis assay, scientists were able to observe the features and properties of DNA replication and test various hypotheses about how the process works.
    • The process of DNA replication was identified by several teams of researchers all working together to break down the process into multiple steps that could more easily be studied individually.

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  • Gene Expression
  • Did you know?

    Did you know that blue eyes are the result of defective genes for pigment? Some recessive traits, like eye color, are harmless, while others are deadly. The way that genes translate into physical traits has to do with the particular enzyme that each type of gene makes, a discovery that was made by two scientists by way of the mutant bread mold they created, winning them the Nobel Prize in 1958.

    Summary

    Through a look at the devastating Tay-Sachs disease and other hereditary conditions, this module explores the connection between genes and enzymes. The role of dominance vs. recessivity is examined. The module traces developments in our understanding of gene expression, starting with a rediscovery of Mendel’s laws of inheritance and built upon by the pioneering work of later scientists. The module introduces the Central Dogma of molecular biology, which is the one-way process of using DNA to make RNA and RNA to make proteins.

    • NGSS
    • HS-C1.5, HS-LS1.A2, HS-LS3.A1, HS-LS3.B1
    Key Concepts
    • Genes cannot be used directly by organisms. The information stored in genes must be used to make products, such as enzymes, that cells need to perform different functions. Gene expression is the chemical pathway from genes to the gene products, such as proteins, that organisms can use.
    • Since organisms have two genes for everything, even If one gene of a pair produces a defective enzyme or no enzyme at all, the other gene in the pair will make enough enzyme to do its job. Only an individual with two genes for a defective enzyme will actually show the recessive trait, such as an inherited disease or condition, blue eyes, or a recessive peapod shape.
    • In the mid-1900s, George Beadle and Edward Tatum showed that a defective gene leads to a defective enzyme. Their “one gene, one enzyme” hypothesis was later expanded to “one gene, one RNA."
    • The genetic code is the set of rules that combines amino acids to form polypeptides and is nearly the same for all life-forms on Earth.
    • The genetic code is not a way for cells to translate genetic information in DNA directly into chains of amino acids to make proteins. Rather, RNA molecules must be made as intermediaries along the way from DNA to the polypeptides that fold into proteins.
    • Genetic information moves in one direction, from DNA to RNA to protein. This is known as the Central Dogma of molecular biology.

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  • Population Genetics
  • Did you know?

    Did you know that elephant seals in the Northern Pacific have a signature asymmetrical face that is extremely rare among other populations of elephant seals? This is because an evolutionary force called a bottleneck event acted upon their gene pool. Other forces can work to change the gene pool of a population, such as natural selection, gene flow, and the founder effect, among others.

    Summary

    Changes in the genetic makeup of a population affect the incidence of certain traits and diseases within the population. Beginning with a look at the abnormally high rate of a dangerous health condition in US Amish communities, this module explores forces that affect a population's gene pool. Among them are natural selection, gene flow, and two types of genetic drift: founder effects and bottleneck events. The Harvey-Weinberg Equilibrium equation is presented along with sample problems that show how to calculate the frequency of specific alleles in a population.

    Key Concepts
    • Variants in genes are called alleles. Alleles can be dominant, meaning they are always expressed, or recessive, meaning that only individuals that receive defective copies from both parents are affected.
    • The work of Gregor Mendel on genes and inherited traits was important in the development of early genetic theories of traits.
    • In a population, the frequencies of alleles (the variations of genes), genotypes (the alleles an individual possesses), and phenotypes (the characteristics an individual expresses due to the alleles) will remain constant, or at equilibrium, unless acted upon by a force.
    • The Hardy-Weinberg Equilibrium equation (p2 + 2pq + q2 = 1) describes how alleles behave in a given population, also known as a population’s gene pool.
    • Genetic drift refers to changes in gene frequencies due to random events, which can happen very quickly, producing dramatic and sudden effects.
    • There are two main types of genetic drift: bottleneck events (when a population suffers a sudden catastrophic decline and is repopulated by a small group of survivors) and Founder effects (when a new population is started by just a few members of the original population).

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